The present invention relates to vehicle heaters that shear viscous fluid with a rotor to generate heat and transmit the heat to a further fluid.
Automobiles are generally provided with hot-water type heaters. In a vehicle having such a heater, engine coolant is heated by the engine. The heater typically has a heater core housed in a duct. The heated coolant is sent to the heater core to warm the passenger compartment. In a diesel engine vehicle or a lean burn engine vehicle, the amount of heat produced by the engine is relatively small. Thus, the amount of heat transmitted to the coolant is small. It is difficult for the coolant to reach a certain temperature such as 80.degree. C. when the amount of heat sent to the heater core is small. Therefore, the heat used to warm the passenger compartment may be insufficient.
To solve this problem, a shearing action heater, which functions as an auxiliary heater, has been proposed. The auxiliary heater is arranged in an engine coolant circulating circuit to heat engine coolant. Japanese Unexamined Patent Publication No. 2-246823 describes a typical shearing action heater. The heater has a housing, which houses a heating chamber and a water jacket (heat exchange chamber), a drive shaft driven by an engine, and a rotor retained in the heating chamber. The rotor rotates integrally with the drive shaft. Viscous fluid (such as high viscosity silicone oil) is contained in the heating chamber. A belt transmission and an electromagnetic clutch connect the engine to the drive shaft. Thus, the engine drives the drive shaft integrally with the rotor. The rotation of the rotor shears the viscous fluid to produce fluid friction and generate heat. The heat raises the temperature of fluid (engine coolant) circulating through the water jacket.
The viscosity of the viscous fluid increases at low temperatures. Thus, when the prior art shearing action heater commences operation (when the engine starts to rotate the rotor) at low temperatures, the high viscosity of the viscous fluid interferes with the smooth rotation of the rotor. In other words, when the heater commences operation under lower temperature conditions, a large load is applied to the engine by way of the rotor, the electromagnetic clutch, and the belt transmission. Therefore, shocks may be produced, slippage may occur in the electromagnetic clutch, or the belt of the belt transmission may slip. These occurrences may produce noise and cause early wear of various components in the auxiliary heater.